Metamorphism and Geochronology of the High Pressure Granulite and Upper Amphibolite Facies Rocks from the Southwest Swedish Granulite Region

The present thesis presents pressure-temperature and geochronological data from the high-grade mafic rocks from the Southwest Swedish Granulite Region, which forms part of the southwestern Baltic Shield. The region is characterized by variably deformed and migmatized granitic to intermediate gneisses, metabasites and charnockites metamorphosed in granulite and upper amphibolite facies.

The peak metamorphic conditions of the mafic granulite facies rocks have been estimated to c. 700-800°C and c. 0.85-1.17 GPa. The P-T estimates from garnet amphibolites are c. 680-750°C and c. 0.85-1.06 GPa, which overlap with, but generally are slightly lower than those of the granulite facies rocks. The recorded pressure corresponds to depths of about 35-40 km. The U-Pb zircon ages of c. 975±20 Ma from two garnet amphibolites are interpreted to approximately date the peak of the high-grade metamorphism in the Southwest Swedish Granulite Region. This age is confirmed by Pb-Pb evaporation ages of c. 990-955 Ma for metamorphic zircons from both samples.

40Ar/39Ar hornblende ages range between 930 and 1030 Ma. The older ages are tentatively interpreted to be associated with crustal thickening, while the younger 40Ar/39Ar ages of c. 930 Ma are interpreted to be associated with the final orogenic uplift. Obtained titanite ages of c. 925-945 Ma (206Pb/238U age) from two garnet amphibolites are suggested to represent either cooling ages of the U-Pb titanite system or formation of titanite during retrogression.

Sm-Nd whole rock and mineral analyses obtained from the granulite facies rocks and garnet amphibolites yield isochron ages between 900 and 945 Ma. These ages are comparable with, or even slightly lower than, the titanite ages and the youngest 40Ar/39Ar hornblende ages. These results suggest that the Sm-Nd mineral system closed at a comparatively low temperature (c. 500°C).

Based on the studies of petrology, geothermobarometry and geochronology, a tentative clockwise P-T-t evolution path of the Southwest Swedish Granulite Region is suggested.